The others have mentioned several things that are wrong about installing a 4 3/4 inch bolt circle wheel on a 120m m bolt circle wheel. I'll see if I can add to their info.
First, even though the wheel bolted up, the bolts are at a slight angle into their respective threaded hole, which puts a side loading on the threads. Structurally, that's a very bad thing, since the threads themselves are a stress riser when an angle like that is present, eventually the bolt will fracture somewhere along the thread, most likely where it enters the hub.
Second, it's not likely that the GM wheels are hubcentric on the BMW hub, and are unlikely to be made hubcentric with adaptor rings. The hubcentric fit takes the major part of the vertical load from the wheel and transfers that load to the suspension, the bolts take the thrust (actually tension) loads and only a small part, if any, of the vertical loads. Without hubcentricity, the bolts are taking the full vertical
AND their intended tension loading.
And last, there's no point in using a GM wheel on your BMW. There is a huge variety of wheels in 15, 16, 17, and even 18 inch diameters that already fit correctly, or can be fitted with 1.5mm hubcentric adapter rings. It's not a case of
sullying your BMW with GM wheels, it simply makes no sense in any meaningful way. :p
